1. Field of the Invention
The present invention relates to a voltage measuring sheet which is capable of qualitatively measuring the voltage output or existing strength of batteries and which has a simplified structure and no moving parts.
2. Description of the Related Art
The extent of the use of small batteries, particularly dry cell batteries has increased rapidly and steadily with the diversified development of electric and electronic products. There is really no end to the list of devices that employ batteries as their energy sources, e.g., portable radios, televisions, cassette decks, tape recorders, etc., timepieces, cameras and various kinds of toy. Batteries are generally stored while being set in such devices. Therefore, there are cases where an electric or electronic device is continuously used without testing the existing strength or condition of the battery set therein, resulting in a failure of operation during use due to lack of voltage. It may be necessary in order to avoid such problem to replace the battery with a new one at the time of contemplated use, which involves a substantial waste of useful battery life.
Testers are generally employed as voltage measuring devices but they are bulky and cumbersome as devices which are frequently used to test the charge state or condition of batteries such as those of the 1.5 and 9 volt dry cell types. Accordingly, almost no testers are actually used for such purposes.
U.S. Pat. No. 4,006,414 discloses a current indicating device wherein a temperature-sensitive coloring liquid crystal layer is provided on an electrically conductive layer carried by a substrate such that the liquid crystal layer senses qualitatively heat which is generated by a part of the conductive layer when a power supply is connected thereto.
The applicant of the present invention has already proposed in Japanese Patent Application No. 93838/1986 and Japanese Utility Model Application No. 67931/1986 a voltage measuring sheet wherein an electrically conductive layer and a temperature-sensitive coloring liquid crystal layer are provided on a substrate, and two electrodes of a battery which is to be measured are brought into contact with two ends, respectively, of the conductive layer to cause portions thereof to generate heat which in turn causes the liquid crystal layer to change in color through a length corresponding to the voltage output or existing strength of the battery under test.
The above-described liquid crystal type voltage measuring sheet suffers from the following problems. Since the liquid crystal layer is coated on a background layer which is printed in dark or black, scale pattern for qualitatively indicating the existing strength of batteries must be printed on the surface of the sheet in such a manner that the pattern is spaced apart from the background and liquid crystal layers and extends parallel to the liquid crystal layer. Accordingly, it is necessary for the user of the device to determine the condition of the battery by checking the length of a portion of the liquid crystal layer which has changed in color against the scale pattern which is spaced apart from the liquid crystal layer.
The present invention aims at eliminating the need to provide the above-described background layer and facilitating the reading of indication of the existing strength or condition of batteries as well as simplifying the display design by printing a scale pattern underneath a temperature-sensitive decoloring type coloring layer which changes color and becomes transparent substantially proportional to the voltage output or existing strength of the battery under test.